Copper alloys are mainly used as an electric contact material for connectors. The formation of a nonconductor or an oxide coating having a high electric resistivity on the surface of a copper alloy causes the risks that contact resistance is increased and a function of the electric contact material is deteriorated.
Therefore, when a copper alloy is used as an electric contact material, there are cases where a layer of noble metal, such as Au or Ag, which is unlikely to be oxidized, is formed on the surface of the copper alloy with a plating treatment or the like. However, it is expensive to form a noble metal layer, and therefore, in general, Sn plating, which is inexpensive and has a relatively high corrosion resistance, is frequently used.
On the other hand, a Sn plating film is relatively soft, and therefore, when provided on the surface of the electric contact material, there is a risk that the Sn plating film is worn out in an early stage, thus causing an increase in the contact resistance. Furthermore, when a terminal is inserted in which the electric contact material provided with the Sn plating film is used, insertion force disadvantageously increases.
In order to deal with these conventional problems, a technique for forming a CuSn-alloy layer on the outermost surface of the electric contact material for a connector (Patent Document 1), a technique for forming a layer of Sn or a Sn alloy on the outermost surface and forming a layer of an alloy containing an intermetallic compound mainly including Cu—Sn thereunder (Patent Document 2), and a technique for forming a Ag3Sn-alloy layer on a Sn-based plating layer (Patent Document 3) have been proposed.
However, with the above-mentioned conventional techniques, the foregoing problems have not been sufficiently solved. Therefore, as a result of intensive research, the inventors developed a method in which, after a layer of an alloy such as NiSn or CuSn is formed on a base material, an insulating oxide layer formed thereon is once removed, and then an oxidizing treatment is performed again. With this method, a layer of a mixed oxide including NiOx (x≠1) and SnOy (y≠1), a layer of a mixed oxide including CuOx (x≠1) and SnOy (y≠1), or a layer of a mixed hydroxide is formed on the surface of the alloy layer. Since the oxide layer or hydroxide layer is conductive and suppresses the oxidation of the alloy layer, the conductivity of an electric contact can be maintained for a long period of time, and low contact resistance can be stably achieved. Since the alloy layer formed on the base material is hard and excellent in wear resistance and has a low friction coefficient, it is possible to made the insertion force sufficiently small when the terminal is inserted (Patent Document 4).